1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of producing the same. More particularly, the invention relates to a liquid crystal display device of the type of vertical alignment in which the liquid crystal molecules are aligned nearly vertically to the surfaces of the substrates when no voltage is applied and a method of producing the same.
2. Description of the Related Art
As the active matrix type liquid crystal display device (LCD), there has heretofore been widely used a liquid crystal display device of a TN (twisted nematic) mode in which a liquid crystal material having a positive dielectric anisotropy is aligned horizontally to the substrate surfaces and being twisted by 90 degrees between the opposing substrates. However, the liquid crystal display device of the TN mode is accompanied by a problem of poor viewing angle characteristics, and study has been conducted extensively to improve the viewing angle characteristics.
There has further been developed a liquid crystal display device of the MVA (multi-domain vertical alignment) system in which a liquid crystal material having a negative dielectric anisotropy is vertically aligned and the tilt directions of the liquid crystal molecules when a voltage is applied are regulated by the protrusions and slits formed on and in the surfaces of the substrates to substitute for the system of the TN mode. The liquid crystal display device of the MVA system features a great improvement in the viewing angle characteristics.
A liquid crystal display device of a general MVA system will be described with reference to FIGS. 20 and 21. FIG. 20 is a schematic perspective view illustrating, in cross section, the liquid crystal display device of the MVA system, and FIG. 21 is a schematic view illustrating the constitution of three pixels in the liquid crystal display device of the MVA system and the direction of alignment of the liquid crystal molecules. In the liquid crystal display device of the MVA system as shown in FIGS. 20A and 20B, the liquid crystal molecules 108 of the liquid crystal material having the negative dielectric anisotropy are aligned nearly vertically to the substrate surfaces between the two pieces of glass substrates 110 and 111. Though not shown, a pixel electrode connected to a thin-film transistor (TFT) is formed for each pixel region on one glass substrate 110, and a common electrode is formed on the whole surface of the other glass substrate 111. Protrusions 120 are formed on the pixel electrode, and protrusions 121 are formed on the common electrode. The protrusions 120 and 121 are alternately arranged. A vertical alignment film that is not shown is formed on the pixel electrodes, common electrode and protrusions 120 and 121.
When the TFT is turned off and no voltage is applied to the liquid crystal molecules 108, the liquid crystal molecules 108 are aligned nearly vertically to the substrate interfaces as shown in FIG. 20A. When the TFT is turned on, a predetermined voltage is applied to the liquid crystal molecules 108 whereby the tilt directions of the liquid crystal molecules 108 are regulated by the structure of the protrusions 120 and 121. Therefore, the liquid crystal molecules 108 are aligned in a plurality of directions as shown in FIG. 20B. For example, when the protrusions 120 and 121 are formed as shown in FIG. 21, the liquid crystal molecules 108 are aligned in four directions A, B, C and D respectively in each pixel. As described above, the liquid crystal display device of the MVA system offers good viewing angle characteristics since the liquid crystal molecules 108 are aligned in a plurality of directions in each pixel when the TFT is turned on.
In the liquid crystal display device of the MVA system, the tilt directions of the liquid crystal molecules 108 are not regulated by the alignment film. Therefore, the MVA system requires no step of alignment treatment such as rubbing that is essential in the horizontally aligned system as represented by the one of the TN mode. This eliminates the problem of static electricity and dust caused by rubbing, and offers an advantage in the process such as eliminating the washing step after the alignment treatment. Another advantage is the display quality without developing the display unevenness that stems from the dispersion in the pre-tilt. As described above, the liquid crystal display device of the MVA system offers such features as simplified production process, improved yield of production bringing about a decrease in the cost, and a high display quality.
However, even the liquid crystal display device of the MVA system that does not require rubbing must be coated with an alignment film. Due to the irregular film thickness and infiltration of foreign matter in the step of printing the alignment film, therefore, the yield of production decreases, the cost of members increases and the tact time lengthens. As described above, the conventional liquid crystal display device of the MVA system still involves a problem of an increase in the cost of production due to the step of printing the alignment film.
In recent years, further, the mother glasses of very large sizes have been produced in response to an increase in the size of the liquid crystal display devices. However, the device for printing the alignment film cannot be easily modified to respond to the mother glasses of very large sizes. In recent years, further, there have been used substrates of the form of thin and flexible films and substrates of shapes that are not flat but are bent. It is quite difficult to print the alignment film on these substrates. Therefore, the liquid crystal display devices using the mother glasses of large sizes and substrates of special shapes are accompanied by a problem of difficulty in obtaining a good liquid crystal alignment.
Patent document 1: JP-A-11-95221
Patent document 2: JP-A-5-232465
Patent document 3: JP-A-8-338993
Patent document 4: JP-A-8-36186